1. Field of the Invention
This invention relates to a liquid crystal composition having a broad mesomorphic range. More particularly, it is concerned with a novel nematic liquid crystal composition for use in display devices, in which the electrical and temperature characteristics thereof are improved.
2. Description of the Prior Arts
In recent years, a number of researches and studies have been made on applications of liquid crystals, wherein the nematic type liquid crystal has been found to be applicable to various kinds of display devices. Such nematic liquid crystal can be classified, depending on its dielectric properties, into an Nn type liquid crystal and an Np type liquid crystal. The Nn type liquid crystal designates a substance having a negative dielectric anisotropy, i.e. the substance having a direction of its electrical dipole moment substantially perpendicular to the molecular long axis, while the Np type liquid crystal designates a substance having a positive dielectric anisotropy, i.e. the substance having a direction of its electrical dipole moment substantially parallel to the molecular long axis.
Since its application to a panel type display device utilizing its dynamic scattering mode (DSM) by Heilmeir of R.C.A. in 1968, the Nn type liquid crystal has attained remarkable progress in its researches and studies, and is now practically used in various fields such as horological instruments, electronic desk calculators, and so forth.
On the other hand, the Np type liquid crystal has first drawn attention of experts in this field by M. Schadt and W. Helfrich of Hoffmann-La Roche in 1971 through their thesis on "Twisted Nematic (T.N.) System" published in "Applied Physics Letters 18, (4), 127, 1971". The method for producing a liquid crystal cell of T.N. system is described in detail in the abovementioned literature as well as laid-open Japanese Patent Application No. 47-11737. Outlining this method, a liquid crystal substance is held between two electrodes, at least one of which is transparent, through a spacer, the electrode surface being so treated as to impart orientation to the liquid crystal, and a liquid crystal cell which has been so oriented that the molecular axis of the liquid crystal oriented on the mutually opposed electrode surfaces may become mutually perpendicular is disposed between two polarization plates. In this cell, when no electric field exists across the electrodes, the liquid crystal molecules are in such a construction that the molecular axis thereof is parallel to the electrode surface and forms a spiral as rotated by 90 degrees from one surface to the other. A linearly polarized light incident on this cell is emitted from the cell by the polarized plane thereof being rotated by 90 degrees so that when the cell is placed between the cross nicols, the sight can be seen bright, and when it is placed between the parallel nicols, the sight is seen dark. When an appropriate voltage is applied to this cell, the long axis of the liquid crystal molecules is oriented substantially perpendicularly with respect to the electrode surfaces with the consequence that the sight is seen dark in the case of the cell being placed between the cross nicols, and it is seen bright, when the cell is placed between the parallel nicols. In this manner, by utilizing contrast between the bright sight and the dark sight, desired display can be performed.
Since suc T.N. cell possesses various advantageous aspects in comparison with the DSM cell such that it has a quick response, can operate at a low voltage, is low in its power consumption, and so forth, its practical utility as the alpha-numeric display devices such as horological instruments, electronic desk calculators, and various measuring instruments is remarkably expanding in recent years.
The liquid crystal substance to be used for such display device is required to have various characteristics. For instance, the liquid crystal substance should have a wide temperature range between a transition temperature T.sub.N of the crystal (or smectic) phase.revreaction.nematic phase and a transition temperature T.sub.I of nematic phase.revreaction.isotropic liquic phase, i.e. a temperature range exhibiting the nematic phase, or mesomorphic range (hereinafter abbreviated as "M.R.") which is in the operating temperature range of, for example, between 0.degree. C. and 50.degree. C. Also, stability of the display device to the environmental conditions such as temperature change, light, humidity, etc., or stability to electro-chemical reactions due to voltage applied to operate the device constitutes the important conditions for the liquid crystal substance. In the field of photographic cameras or automotive vehicles, since the operating temperature of the device could range between -30.degree. and 80.degree. C. or higher, or, in some occasion, higher than 100.degree. C., it is desirable that the liquid crystal be not thermally decomposed, and the molecular orientation of the liquid crystal which is characteristic of the T.N. system be not disturbed, even under such conditions. Furthermore, good response property to the voltage applications is required for such liquid crystal. In general, the response property of the liquid crystal to the voltage application depends on viscosity or dielectric anisotropy of the liquid crystal substance. However, since the liquid crystal substance is generally highly viscous, its responsiveness at time of voltage application or its elimination, in particular, at the time of decaying due to the voltage elimination, is not so satisfactory. Accordingly, a liquid crystal substance of as low viscosity and as large dielectric anisotropy as possible is desired.
Or, in driving the display device, a large "voltage margin" to be described later is also the requisite condition for the liquid crystal substance. In other words, when the alpha-numeric display is to be effected, there are two ways of "static drive" and "dynamic drive" from the standpoint of the lead wire connection. In case many numbers of letters or characters are to be displayed, the dynamic drive system is more advantageous. This dynamic drive is exactly the same in theory as the matrix drive as shown in FIG. 1 of the accompanying drawing. In general, the liquid crystal cell is applied with a voltage in such a manner as shown in FIG. 1 to prevent cross-talk from taking place. In the drawing, the vertical lines and the horizontal lines representing the electrodes are mutually intersected, although, in the actual cell, both lines are spatially separated, and the liquid crystal substance is filled therebetween. The solid black dots in the drawing denote the selection points, to which a voltage V is applied, while the white dots denote the semi-selection or non-selection points, to which a voltage 1/3V is applied. The voltage V is so determined that light may be intercepted at this solid black dot portions, and it may be transmitted through the white dot portions.
FIG. 2 indicates a relationship between voltage and intensity of transmitted light, or transmission. If a voltage V can be used which is sufficiently larger than a voltage value V.sub.s at the time when the transmitted light intensity becomes saturated, the contrast, etc. do not matter so much. However, since undesirable cross-talk arises as mentioned in the foregoing, there exists an upper limit voltage V.sub.b from this point. Also, since a definite transmitted light intensity should be obtained, there is necessity for a lower limit voltage V.sub.a. In other words, when the liquid crystal cell is subjected to the matrix drive, it is so designed that the source voltage may satisfy a relationship of V.sub.a .ltoreq.V "V.sub.b. However, in case the value of the "voltage margin" (V.sub.b-V.sub.a) is small (in most of the heretofore known liquid crystal substance, the value was 0.1 to 0.2V or so), a circuit that is able to confine this driving voltage constantly within this range becomes necessary with the consequence that the circuit becomes highly complicated or expensive in manufacturing costs. In addition, since the voltage margin is largely affected by a temperature change, a usable temperature range is limited, or there is also required a circuit in which a drive voltage is varied in correspondence to the temperature change. Such situation is shown in FIGS. 3A and 3B, FIG. 3A indicating a relationship between the temperature and the voltage margin within the mesomorphic range (M.R.), and FIG. 3B showing typical example of an ordinary nematic liquid crystal. In FIG. 3A, the liquid crystal composition is considered excellent as the diagonally hatched portion S in the graphical representation becomes larger, when the driving temperature range of the liquid crystal cell is set between T.sub.1 and T.sub.2. If the lower limit voltage V.sub.a at T.sub.1 is greater than the upper limit voltage V.sub.b at T.sub.2 as shown in FIG. 3B, there is no portion at all corresponding to the above-mentined hatched portion S in FIG. 3A. On the other hand, if preference is given to the driving temperature range T.sub.1 -T.sub.2, the circuit should be so designed that voltage may be varied by the temperature. Further, when the voltage is made constant, the operating temperature range of the device utilizing the cell should be restricted with the consequence that circulation area of the article is limited.
In order to meet the afore-described demands, there have so far been studied various kinds of liquid crystal substances.
U.S. Pat. No. 3,952,046 discloses a liquid crystal substance represented by the following general formula: ##STR3## (where R represents alkyl having a carbon content of from 3 to 8, or alcoxy group having a carbon content of from 3 to 10).
U.S. Pat. No. 3,954,653 discloses an Np type liquid crystal composition consisting of compounds represented by the following general formulas: ##STR4## (where R is selected from C.sub.n H.sub.2n+1 O, C.sub.n H.sub.2n+1 COO and C.sub.n H.sub.2n+1 OCOO groups in straight chain form, n being an integer of from 1to 10).
U.S. Pat. No. 4,011,173 teaches an Np type liquid crystal composition consisting of a mixture of the following compounds: ##STR5## (where R.sub.1 and R.sub.2 are alkyl or alkoxy group having a carbon content of from 1 to 8); and EQU X--R--Y
(where X is dialkylamino having a carbon content of from 2 to 4; Y is Cn, NO.sub.2 or CF.sub.3 ; and R is ##STR6## The patent also discloses that the following compound also be mixed depending on necessity: ##STR7## (where R.sub.1 and R.sub.2 are alkyl or alcoxy group having a carbon content of from 1 to 8).
U.S. Pat. No. 4,020,002 discloses a liquid crystal composition prepared by mixing the following compound as the Nn type liquid crystal: ##STR8## with the following compound as the Np type liquid crystal ##STR9## at a ratio of from 2 to 50 wt %.
Further, U.S. Pat. No. 3,891,307 discloses an electro-optical device, in which a liquid crystal substance is held between the opposed electrodes. For the liquid crystal substance, there is described a composition consisting of 30 to 99% by weight of Np type nematic liquid crystal and 70 to 1% by weight of an optically active steroid compound. Further, in this patent specification, the Np type liquid crystal is generally exemplified as Schiff base having CN group, phenyl benzoic acid ester, azo-type compounds, and so on, while the Nn type liquid crystal is generally exemplified as azo-type and azoxy-type compounds which have alkyl and alkoxy, and Schiff base.
In most of the heretofore known liquid crystal compositions, it was considerably difficult to satisfy the aforedescribed various conditions. For instance, there have been obtained thermally stable compositions among these known liquid crystal compositions although their transition temperature T.sub.N at the low temperature side increases as the temperature T.sub.I increases, hence few liquid crystal compounds could cover the whole practical temperature range. Furthermore, those liquid crystal compositions which can satisfy the above-mentioned conditions and possesses satisfactory response to the voltage application or removal are also few. In addition, very few liquid crystal composition could show stable electrical characteristics against temperature change, e.g. small fluctuations in the voltage margin.